docs: def _verify_forward_declarations_used(self, forward_declarations,
                                            decl_uses, file_uses):
          """Find all the forward declarations that are not used."""
          for cls in forward_declarations:
              if cls in file_uses:
                  if not decl_uses[cls] & USES_DECLARATION:
                      node = forward_declarations[cls]
                      msg = ("'{}' forward declared, "
                             'but needs to be #included'.format(cls))
                      self._add_warning(msg, node)
              else:
                  if decl_uses[cls] == UNUSED:
                      node = forward_declarations[cls]
                      msg = "'{}' not used".format(cls)
                      self._add_warning(msg, node)